Pectin is a heteropolysaccharide which is found in the form of a water insoluble parent pectic substance--protopectin--in the primary cell wall and the middle lamella of green land plants. Pectin is the generic designation for compounds which result from restricted hydrolysis of protopectin.
The exact nature of protopectin is not completely understood. It is, however, generally recognised that protopectin is a complex structure in which pectin is attached to other cell wall components, such as cellulose and hemicellulose, by covalent bonds, hydrogen bonds and/or ionic interactions.
Pectin is a linear polymer composed of D-galactopyranosyl uronic acid units which are linked through .alpha.-1.4-glycosidic bonds forming long chains of polygalacturonic acid. Part of the carboxyl groups of the uronic acid units are esterified with methanol. In the plants the residual carboxyl groups are partly or completely neutralised with cations of calcium and magnesium which inherently are contained in the plant tissues.
The heteropolysaccharide nature of pectin derives from the fact that other sugars are incorporated in the pectin molecule. The most common sugars are L-rhamnose, inserted by .alpha.-1.2-linkages in the galacturonan backbone, and .beta.-D-xylose, attached as single-unit side chains mainly to O-3 of the galactopyranosyl uronic acid residues in the backbone, and D-galactose and L-arabinose, which occur in long side chains, only attached to rhamnopyranosyl residues.
Major sources of commercial pectin products are citrus peel and apple pomace in which protopectin represents 10-40% by weight of the dry matter.
Pectin in isolated and more or less purified form is mostly obtained from the plant material by treating the material with an acid or base under suitable conditions whereby the protopectin is split, followed by extraction of the water soluble pectin. In conventional industrial production of pectin, acid or base treated pectin-containing plant starting material is subjected to extraction with dilute acid which step is followed by separating the depectinated plant material by e.g. filtration or centrifugation to obtain a viscous extract containing up to 1% by weight of soluble pectin. This extract may be purified and processed further. Finally, the pectin is precipitated, separated and dried to obtain a commercial pectin product which is at least partially purified.
The hydrolysis of the polygalacturonic acid methyl esters occurs at both acidic and alkaline pH values, but dominates at pH values above 7, in particular above 9. The polygalacturonic acid methyl ester groups are also hydrolysed by pectin esterases of plant origin. Pectin esterases are present in plant materials such as citrus fruits. Contrary to chemical hydrolysis which attacks the polygalacturonic acid methyl ester groups at random, the pectin esterases attack next to a free carboxyl group and proceed along the backbone thus creating a blockwise or non-random distribution of free carboxyl groups.
For water soluble pectin resulting from hydrolysis of protopectin average values of 100,000 to 200,000 for the molecular mass have been measured.
In the course of time, several methods of extracting pectic substances from vegetable matter, such as i.a. citrus peel, apple pomace or beet pulp have been described. These known processes aim at extracting substantially all of the pectin content of the pectin-containing starting material.
Thus, as one example U.S. Pat. No. 2,008,999 discloses a method of producing pectin comprising subjecting a pectin-containing material to an extraction at a pH of from 0.7 to 2.2 with a solution of a strong inorganic acid at a temperature of from 50 to 75.degree. C. until the pectin in a test sample is precipitable by calcium ions at a pH of from 3 to 7, filtering the extraction mixture and recovering the pectin from the filtrate which has been brought to a pH of 3-7.
U.S. Pat. No. 2,273,521 describes a process which comprises treating a pectin-containing material with a solution of a solvent in which pectin is insoluble and an inorganic or organic acid at a temperature of about 70.degree. C. and separating the extraction residue containing the water-soluble pectin. The pectin is recovered from the residue by extraction with water and isolated in solid form from the extract.
Other processes of preparing bulk-extracted pectin are disclosed in U.S. Pat. No. 2,586,407, EP 688,792-A, U.S. Pat. No. 4,016,351 and U.S. Pat. No. 2,020,572.
Thus, the prior art describes methods of producing isolated pectin wherein a pectin-containing material is subjected to an exhaustive, i.e. a substantially complete extraction and the pectin is recovered as a whole, i.e. the pectin is provided as bulk-extracted pectin. Depending on the processing conditions including pH, temperature and time of extraction it is, to some extent, possible to control such bulk extraction processes so that pectins having different degrees of esterification are obtained.
However, the characteristics of such extracted pectin products can be defined only in a statistical manner because of the variability in polymerisation, methoxylation and acetylation, neutral sugar content, and distribution of substituents along the backbone. It is reasonable to assume that not two pectin molecules in a bulk-extracted pectin preparation are identical. Thus, the chemical and physical properties of a single pectin molecule are determined i.a. by the molecular mass of the molecule and the concentration and distribution of free and esterified carboxyl groups in the molecule as well as the concentration and the distribution of other groups in the molecule. Thus, two pectin molecules having the same molecular mass and the same esterification may interact differently with another molecule, particle or ion.
The degree of esterification (DE) of the pectic substances is of considerable significance for the food additive effects of pectin containing products. Typically, pectin-containing products have a DE which is in the range of 10-90%. In particular foods it may be advantageous to use pectin having a high DE, e.g. more than 50%.
High-esterified bulk pectin which has been extracted under conditions selected so as to obtain bulk-extracted pectin preparations having specific functional properties often exhibits an undesired performance such as giving rise to a haze in gels, enhanced viscosity resulting in pregelation, incomplete solubility, and syneresis in milk systems, such as yoghurt.
This undesired performance which is observed with bulk-extracted pectin is attributed to the molecular variability and a varied interaction between calcium ions and separate pectin molecules having varying affinity towards calcium ions and other charged particles such as for example proteins.
Bulk-extracted pectin products comprise molecules that represent a broad distribution of methoxylation degree and molecules of non-random as well as random methoxylation pattern. This affects the important quality parameters for pectin, such as setting time, breaking strength of gels, setting temperature profile and solubility of pectin in food manufacturing applications.
An important objective of the present invention is therefore to provide a method for preparing separate fractions of high-esterified pectin having improved functional characteristics over bulk-extracted pectin.
However, it has surprisingly been found that such selected pectin fractions are also very suitable starting materials for making derivatized pectin fractions having exceptionally good functional characteristics. Thus, by subjecting the high-esterified pectin fractions according to the invention to treatment with an acid and/or ammonia under deesterifying conditions, deesterified and optionally amidated pectin fractions can be obtained which are useful as additives in various food applications such as for example fruit preparations and dairy products. It has been found, that such deesterified pectin fractions have improved functional characteristics both in comparison with the high-esterified pectin starting material and similarly treated bulk-extracted pectin.